The metabolism of human procollagen is being studied with respect to: the enzymatic activities which convert procollagen to collagen; the nature of the interaction which binds procollagens and collagens to the fibroblast plasma membrane; and the functions subserved by collagenous molecules bound to the fibroblast surface. A radioimmunoassay for human type I procollagen is being applied to the sera and body fluids of patients with diseases affecting connective tissues.